This application addresses nondestructive materials characterization and, particularly fabrication of reference samples for validating inspection methods and for establishing correlations between sensor responses and actual material condition. The nondestructive material characterization provides a quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks, cracks or stress variations in magnitude, orientation or distribution.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of the prescribed frequency. This excitation produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the test material can be deduced from measurements of the impedance between the primary and secondary windings. Scanning of eddy-current sensors across the material surface is then traditionally used to detect flaws, such as cracks.
For fatigue inspection applications where cracks need to be detected and/or sized, typical nondestructive evaluation (NDE) or inspection (NDI) methods use reference standards or samples based on EDM notches. Some standards have only the notches themselves while other standards have fatigue cracks grown from EDM notches with the original notch removed, such as the engine structural integrity program (ENSIP) standards. However, there can be differences in morphology, shape, and distribution compared to cracks grown under service conditions so that they may not be sufficient to demonstrate detection reliability for relatively small crack sizes in engine or structural components. This is especially true for shot peened or otherwise cold worked surfaces, where creating the EDM notch itself can significantly alter the material properties, such as the residual stress distribution in a cold worked surface.